1. Field of the Invention
The present invention relates to a sheet conveying apparatus provided in an image forming apparatus, and more in particular, it relates to a constitution for correcting inclination (or skew) of the sheet to be conveyed.
2. Related Background Art
Conventionally, in general, an image forming apparatus and an image reading apparatus such as a copying machine, a printer, a facsimile, a scanner and the like comprise a sheet conveying apparatus for conveying a sheet such as a recording paper and an original to an image forming portion and an image reading portion. This sheet conveying apparatus is sometimes provided with correction means for performing a skew conveying correction of the sheet and a positional deviation correction of the sheet to align the attitude and position of the sheet before being conveyed to the image forming portion and the image reading portion.
Here, as for a correction system of such correction means, there are those using a registration roller pair, and for example, in the case of the image forming apparatus, a so-called loop registration system has become a mainstreamer, wherein a leading edge of the sheet is abutted against a nip of the registration roller pair at rest so as to cause a deflection on the sheet, and the leading edge of the sheet is allowed to run parallel with the roller nip by elasticity of the sheet so as to perform a correction of the skew conveying, and after that, the registration roller pair is rotated at a predetermined timing so as to establish a synchronization between the sheet and the image.
However, in such a loop registration system, a loop space for forming a loop has been certainly required, thereby making the apparatus large-sized. Further, when the loop space is not sufficiently secured, there is a problem in that a jam (sheet jam) due to a buckling particularly in the sheet such as a thin paper being weak in rigidity has occurred, and abnormal sound (so-called loop sound) has been emitted when the sheet was abutted against the registration roller pair.
Furthermore, there is a problem in that a skew conveying correction capability ends up being changed depending on strength of the sheet rigidity. To be more specific, in the case of the thin paper, which is weak in rigidity, the leading edge of the sheet lacks an abutting force when abutting against the registration roller nip, and there are some cases where the leading edge of the sheet does not sufficiently abut against the registration roller pair, and in this case, it is totally impossible to perform the skew conveying correction.
Further, in the case of a thick paper and the like which is strong in rigidity, there is a problem in that the sheet ends up going through the nip of the registration roller pair due to an impact caused by abutting against the nip of the registration roller. To prevent this from occurring, for example, when an attempt is made to give a load and the like to the registration roller pair by a brake member, it invites a cost-up of the product.
Still further, when there is a curl or a crease on the leading edge of the sheet, the leading edge of the sheet is unable to precisely run along the nip portion of the registration roller, and as a result, there arises a problem in that a precise correction of the skew conveying cannot be performed, thereby lowering a print accuracy.
On the other hand, in recent years, the image forming apparatus and the image reading apparatus have been digitalized to such an extent that, after an original is read once, its image information can be electrically encoded and stored in a memory. At an image forming time, the information inside the memory is read, and an image corresponding to the image information of the original is allowed to be formed on a photosensitive body by an exposing apparatus such as a laser beam, a LED array and the like. Hence, even in a plurality of copies, no mechanical movement such as an optical system is needed.
This has made it possible that a sheet interval, which is an interval between the sheets, is closed up, and a number of sheets are processed within a shortest possible time. As a result, for example, in the case of the image forming apparatus, at an image forming time, the substantial improvement of an image forming speed has come to be attempted without increasing a processing speed.
Nevertheless, in case of using an apparatus adopting the loop registration system described as above as the sheet conveying apparatus, a sheet is stopped once so as to form a loop, and therefore, a sheet interval is inevitably decided, thereby greatly affecting the improvement of the image forming speed (productivity).
Consequently, to overcome such a defect, a sheet conveying apparatus adopting the registration system so as to automatically reform the skew conveying of the sheet has been disclosed in Japanese Patent Application Laid-Open No. 10-067448.
Here, this sheet conveying apparatus comprises a conveying roller pair (registration rollers) for nipping and conveying the sheet, a sensor for detecting an amount of skew of the sheet provided at the downstream side in a conveying direction of the conveying roller, and correction means for correcting the inclination of the conveying roller such that the conveying roller is displaced so as to incline in a direction orthogonal to the conveying direction of the sheet, and in case of correcting the skew conveying of the sheet, based on the information of the skew amount detection sensor, the skew conveying of the sheet is corrected by displacing the conveying roller according to the inclination of the sheet.
However, in such a conventional sheet conveying apparatus for correcting the skew conveying of the sheet by displacing the conveying roller, at the time when the conveying roller is displaced so as to correct the skew conveying of the sheet, in the case where the leading edge of the sheet is nipped by the roller positioned at the downstream portionside in a conveying direction of the conveying roller or the tailing edge portion of the sheet is nipped by the roller positioned at the upstream side in the conveying direction of the conveying roller, for example, if the conventional sheet conveying apparatus is applied to the image forming apparatus, there occurs the tugging of the sheet between the rollers, and this causes problems such as the occurrence of a crease on the sheet, the correction of the skew conveying not adequately performed, the appearance of the distortion in the image, and the print accuracy remarkably poor.